3GPP-LTE (3rd Generation Partnership Project Radio Access Network Long Term Evolution, hereinafter, referred to as “LTE”) adopts OFDMA (Orthogonal Frequency Division Multiple Access) as a downlink communication scheme and adopts SC-FDMA (Single Carrier Frequency Division Multiple Access) as an uplink communication scheme (e.g., see Non-Patent Literatures 1, 2 and 3).
In LTE, a radio communication base station apparatus (hereinafter abbreviated as “base station”) assigns resource blocks (RBs) in a system band to a radio communication terminal apparatus (hereinafter abbreviated as “terminal”) for each time unit called “subframe” and thereby performs, communication.
Furthermore, the base station transmits downlink control information (L1/L2 control information) for reporting results of resource assignment to downlink data and uplink data to the terminal. This downlink control information is transmitted to the terminal using a downlink control channel such as a PDCCH (Physical Downlink Control Channel). Here, each PDCCH occupies a resource made up of one or a plurality of consecutive CCEs (Control Channel Elements). In LTE, one of 1, 2, 4 and 8 is selected as the number of CCEs (CCE aggregation level) occupied by the PDCCH according to the number of information bits of downlink control information or a channel state of the terminal. In LTE, a frequency band having a width of a maximum of 20 MHz is supported as a system bandwidth.
Furthermore, when assigning a plurality of terminals to one subframe, the base station simultaneously transmits a plurality of PDCCHs. At this time, the base station includes a CRC bit which is masked (or scrambled) with a destination terminal ID to identify a destination terminal of each PDCCH in the PDCCH and transmits the PDCCH. The terminal demasks (descrambles) CRC bits in a plurality of PDCCHs which are likely to be addressed to the own terminal with the terminal ID of the own terminal and thereby blind-decodes the PDCCHs and detects the PDCCH addressed to its own terminal.
Furthermore, downlink control information transmitted from the base station is called “DCI (Downlink Control Information)” and includes information on resources assigned by the base station to the terminal (resource assignment information) and MCS (Modulation and channel Coding Scheme) or the like. The DCI has a plurality of formats. That is, examples of such formats include uplink format, downlink MIMO (Multiple Input Multiple Output) transmission format, downlink non-contiguous band assignment format. The terminal needs to receive both downlink assignment control information (downlink related assignment control information) and uplink assignment control information (uplink-related assignment control information). The downlink assignment control information has a plurality of formats (downlink assignment control information formats) and the uplink assignment control information has one format (uplink assignment control information format).
For example, for the downlink control information (DCI), formats of a plurality of sizes are defined depending on a transmitting antenna control method and a resource assignment method of the base station or the like. Of the plurality of formats, a format of downlink assignment control information for assigning contiguous bands (hereinafter referred to as “contiguous band assignment downlink format”) and a format of uplink assignment control information for assigning contiguous bands (hereinafter simply referred to as “contiguous band assignment uplink format”) have the same size. These formats (DCI formats) include type information (e.g., 1-bit flag) indicating a type of assignment control information (downlink assignment control information or uplink assignment control information). Thus, even when the size of the contiguous band assignment downlink format and the size of the contiguous band assignment uplink format are the same, the terminal can identify which of the downlink assignment control information or uplink assignment control information is the assignment control information by checking the type information included in the assignment control information.
For example, the contiguous band assignment uplink format is called “DCI format 0” (hereinafter referred to as “DCI 0”), whereas the contiguous band assignment downlink format is called “DCI format 1A” (hereinafter referred to as “DCI 1A”). As described above, DCI 0 and DCI 1A have the same size and can be distinguished by type information. Thus, DCI 0 and DCI 1A will be collectively described as DCI 0/1A below.
Furthermore, in addition to the contiguous band assignment downlink format and contiguous band assignment uplink format, there are DCI formats such as: a format of downlink assignment control information for performing non-contiguous band assignment (“non-contiguous hand assignment downlink format”: DCI format 1: DCI 1); and a format of downlink assignment control information for assigning spatially multiplexed MIMO transmission and (“spatially multiplexed MIMO downlink format”: DCI format 2, 2A: DCI 2, 2A). Here, DCI 1, 2 or 2A is a format used dependently on a downlink transmission mode of the terminal (non-contiguous band assignment or spatially multiplexed MIMO transmission). That is, DCI 1, 2 or 2A is a format set for each terminal. On the other hand, DCI 0/1A is a format that can be used for a terminal in any transmission mode, independently of the transmission mode. That is, DCI 0/1A is a format used in common to all terminals. Furthermore, when DCI 0/1A is used, 1-antenna transmission or transmission diversity is used as a default transmission mode.
Furthermore, studies are being carried out on a method of limiting CCEs to be blind-decoded for each terminal for the purpose of reducing the number of times blind decoding is performed to reduce the circuit scale of the terminal. This method limits a CCE region that can be blind-decoded by each terminal (hereinafter referred to as “search space”). Here, the unit of a CCE region assigned to each terminal (that is, equivalent to the unit for blind decoding) is called “downlink control information assignment region candidate (PDCCH assignment region candidate)” or “blind decoding region candidate.”
In LTE, a search space is set randomly for each terminal. The number of CCEs configuring this search space is defined for each CCE aggregation level of a PDCCH. For example, the number of CCEs configuring a search space is 6, 12, 8, 16 in correspondence with CCE aggregation level 1, 2, 4, 8 of the PDCCH. In this case, the number of blind decoding region candidates is 6 candidates (6=6÷1), 6 candidates (6=12÷2), 2 candidates (2=8÷4), 2 candidates (2=16÷8) in correspondence with CCE aggregation level 1, 2, 4, 8 of the PDCCH. That is, the blind decoding region candidates are limited to a total of 16 candidates. Thus, each terminal needs only to perform blind decoding on a blind decoding region candidate group within a search space assigned to its own terminal, and can thereby reduce the number of times blind decoding is performed. Here, a search space of each terminal is set using a terminal ID of each terminal and a hash function, which is a function that performs randomization. This terminal-specific CCE region is called a specific region (UE specific search space: UE-SS).
On the other hand, the PDCCH also includes control information for assignment of data common to terminals simultaneously reported to a plurality of terminals (e.g., assignment information relating to a downlink broadcast signal and assignment information relating to a paging signal (hereinafter referred to as “control information addressed to a common channel”). To transmit control information addressed to a common channel, a CCE region common to all terminals that should receive a downlink broadcast signal (hereinafter referred to as “common region (common search space: C-SS)”) is used for the PDCCH. In the search space of C-SS, there are a total of 6 blind decoding region candidates; 4 candidates (4=16÷4) and 2 candidates (2=16÷8) for CCE aggregation levels 4 and 8 respectively.
Furthermore, in the UE-SS, the terminal performs blind decoding on DCI formats of two sizes; a first type DCI format (DCI 0/1A) used in common for all terminals and a second type DCI format (DCI 1, 2, 2A or the like) which is dependent on a transmission mode. For example, in the UE-SS, the terminal performs blind decoding on 16 blind decoding region candidates relating to the above-described first type DCI format (DCI 0/1A) and the second type DCI format (DCI 1, 2, 2A or the like) of different sizes respectively and thus performs blind decoding a total of 32 times. Furthermore, in the C-SS, the terminal performs blind decoding on the above-described six blind decoding region candidates for DCI format 1C which is a common channel assignment format (hereinafter referred to as “DCI 1C”) and DCI 1A respectively, and thus performs blind decoding a total of 12 times. Therefore, the terminal performs blind decoding a total of 44 times per subframe.
Here, DCI 1A used to assign a common channel and DCI 0/1A used to assign terminal-specific data have the same size, but the two are distinguished from each other by a terminal ID. Therefore, the base station cart also transmit DCI 0/1A to assign terminal-specific data using the C-SS without increasing the number of times the terminal performs blind decoding.
Furthermore, standardization of 3GPP LTE-Advanced (hereinafter referred to as “LTE-A”), which realizes faster communication than LTE has started. LTE-A realizes downlink transmission rate of a maximum of 1 Gbps or above and an uplink transmission rate of a maximum of 500 Mbps or above. Thus, base stations and terminals communicable at a wideband frequency of 40 MHz or above (hereinafter referred to as “LTE-A terminals”) are expected to be introduced. Furthermore, an LTE-A system is required to accommodate not only LTE-A terminals but also terminals supporting an LTE system (hereinafter referred to as “LTE terminals”).
LTE-A proposes a carrier aggregation scheme that aggregates a plurality of frequency bands for communication to realize wideband communication of 40 MHz or above (e.g., see Non-Patent Literature 1). For example, a frequency band having a width of 20 MHz is considered as a base unit (hereinafter referred to as “component carrier” (CC)) of communication bands. Thus, LTE-A realizes a system bandwidth of 40 MHz by aggregating two component carriers, for example. Furthermore, both an LTE terminal and an LTE-A terminal are accommodated in one component carrier. In the following description, a component carrier in an uplink will be referred to as an “uplink component carrier” and a component carrier in a downlink will be referred to as a “downlink component carrier.”
In LTE-A, studies are being carried out on supporting carrier aggregation using at least five CCs as the system, but the number of CCs actually used differs from one terminal to another depending on a reception capability corresponding to the number of CCs and a required transmission rate of each terminal. Thus, which CC is used is set (configured) for each terminal. A set CC is called “UE CC set.” Furthermore, the UE CC set is semi-statically controlled by a required transmission rate of the terminal.
in LTE-A, when data is assigned to a plurality of component carriers for a certain terminal, assignment control information is reported using a plurality of PDCCHs. That is, a result of resource assignment of each component carrier is reported using one PDCCH per one component carrier. Here, as a method of reporting resource assignment information of each component carrier from a base station to a terminal, assigning data of a different component carrier using a PDCCH transmitted by a certain component carrier, i.e., “cross-carrier scheduling” is under study in LTE-A. To be more specific, studies are being carried out on instructing a component carrier to which the PDCCH is assigned using a carrier indicator (CI) in the PDCCH. That is, each component carrier is labeled by the CI. The CI is transmitted in a field in the PDCCH called “CIF (Carrier Indicator Field)” (e.g., see Non-Patent Literature 5).
Furthermore, LTE-A will newly introduce a transmission method using non-contiguous band assignment and a transmission method using MIMO as uplink transmission methods. Along with this, a definition of new DCI formats (e.g., DCI formats 0A, 0B: DCI 0A, 0B) is under study (e.g., see Non-Patent Literature 4). That is, DCI 0A, 0B is a DCI format which is dependent on an uplink transmission mode. In this case, in addition to the above-described blind decoding of LTE, 16 times of blind decoding are further added in the UE-SS and the terminal performs blind decoding a total of 60 times per subframe.